Catadioptric or compound telescopes use both lenses and mirrors. The most popular design is the Schmidt-Cassegrain, which burst onto the market in the 1970s and quickly gained a place for itself alongside refractors and reflectors, which had been around for centuries. The following comments apply primarily to Schmidt-Cassegrains.
The pluses of the "Schmidt-Cass" are in portability, convenience, and special options such as advanced tracking and electronics -- not visual performance. While most people can haul an 8" reflector in and out of doors, it is awkward and heavy. Most 8" Schmidt-Cassegrains come in a padded footlocker that can be hoisted with one hand. (The tripod is separate.) The footlocker can be stowed in a car trunk or closet like a large piece of luggage, whereas a reflector tends to displace everything around it.
A Schmidt-Cassegrain's relatively short tube allows a motorized mounting to track the stars more reliably, making astronomical photography less difficult (it's never easy). These are excellent photographic telescopes. Elaborate electronic drive controls are available as options on Schmidt-Cass mountings for photographers and CCD camera users. Some can be bought with robotic computerized pointing capabilities.
The image formed by a Schmidt-Cassegrain will probably be a touch less sharp than the image formed by a good reflector of the same aperture. This is most noticeable when observing planets. The cost is higher than for a reflector of the same aperture. A right-angle mirror (star diagonal) is generally used at the eyepiece to provide a comfortable viewing position, and this means your view is mirror-reversed. The focusing mechanism can be a bit sloppy and imprecise. You can't take the scope apart yourself; major adjustments mean shipping it back to the factory.
Catadioptric telescopes are essentially a combination of a refractor and a reflector. There are 2 very popular flavors widely available in today's telescope marketplace. One is the Schmidt-Cassegrain, and the other is the Maksutov-Cassegrain (see images below). These telescopes fold the light path 3 times allowing for a much shorter tube. Because of their clever use of corrective lenses and lack of a spider to hold the secondary mirror, they are free of many of the optical defects present in refractors and reflectors.
Light enters the front of the tube through a corrective lens. The light then travels down the tube to the primary mirror. From there, it is reflected up the tube to a secondary mirror which in turn reflects the light back down the tube to the focal plane.
The focusing mechanisms of Catadioptric telescopes are different than in reflectors and refractors. Instead of moving the eyepiece in and out of the focal plane, the whole primary mirror is moved in and out. Because of their more complex design, their tube construction and mount are generally very well thought out and implemented. Most of them are wonderful telescopes.
Catadioptric telescopes can be very expensive, but a well-built Catadioptric telescope is quite simply a joy to own and use.
Advantages
The pluses of the "Schmidt-Cass" are in portability, convenience, and special options such as advanced tracking and electronics -- not visual performance. While most people can haul an 8" reflector in and out of doors, it is awkward and heavy. Most 8" Schmidt-Cassegrains come in a padded footlocker that can be hoisted with one hand. (The tripod is separate.) The footlocker can be stowed in a car trunk or closet like a large piece of luggage, whereas a reflector tends to displace everything around it.
A Schmidt-Cassegrain's relatively short tube allows a motorized mounting to track the stars more reliably, making astronomical photography less difficult (it's never easy). These are excellent photographic telescopes. Elaborate electronic drive controls are available as options on Schmidt-Cass mountings for photographers and CCD camera users. Some can be bought with robotic computerized pointing capabilities.
Disadvantages
The image formed by a Schmidt-Cassegrain will probably be a touch less sharp than the image formed by a good reflector of the same aperture. This is most noticeable when observing planets. The cost is higher than for a reflector of the same aperture. A right-angle mirror (star diagonal) is generally used at the eyepiece to provide a comfortable viewing position, and this means your view is mirror-reversed. The focusing mechanism can be a bit sloppy and imprecise. You can't take the scope apart yourself; major adjustments mean shipping it back to the factory.
Or
Catadioptric telescopes are essentially a combination of a refractor and a reflector. There are 2 very popular flavors widely available in today's telescope marketplace. One is the Schmidt-Cassegrain, and the other is the Maksutov-Cassegrain (see images below). These telescopes fold the light path 3 times allowing for a much shorter tube. Because of their clever use of corrective lenses and lack of a spider to hold the secondary mirror, they are free of many of the optical defects present in refractors and reflectors.
Light enters the front of the tube through a corrective lens. The light then travels down the tube to the primary mirror. From there, it is reflected up the tube to a secondary mirror which in turn reflects the light back down the tube to the focal plane.
The focusing mechanisms of Catadioptric telescopes are different than in reflectors and refractors. Instead of moving the eyepiece in and out of the focal plane, the whole primary mirror is moved in and out. Because of their more complex design, their tube construction and mount are generally very well thought out and implemented. Most of them are wonderful telescopes.
Catadioptric telescopes can be very expensive, but a well-built Catadioptric telescope is quite simply a joy to own and use.
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